In the field of wireless communication, the terms “User Equipment, UE” and “wireless device” are commonly used for various communication entities e.g. including mobile telephones, tablets and laptop computers. In this disclosure, “wireless device” will be used to represent any wireless communication entity capable of communicating radio signals with a wireless network. It should be noted that a wireless device in this context may also be a Machine Type Communication, MTC, device such as a sensor, counter or measuring device arranged to operate automatically and send reports or other messages to some central node.
Further, the term “network node” represents any node of a wireless network that is arranged to communicate radio signals with wireless devices. Throughout this disclosure, the term network node is interchangeable with base station, transmission point, radio node, eNodeB or eNB, and the term wireless device is interchangeable with UE.
Long Term Evolution, LTE, is a mobile broadband wireless communication technology defined by the third Generation Partnership Project, 3GPP. According to LTE, radio signals are transmitted from network nodes (referred to as eNodeBs or eNBs by 3GPP) to wireless devices (referred to as UEs by 3GPP) over a radio link using Orthogonal Frequency-Division Multiplexing, OFDM. In a wireless network, a Time Division Duplex, TDD, configuration of subframes, also known as “uplink-downlink configuration”, may be used for uplink and downlink transmissions in cells where consecutive subframes are comprised in a repeatable radio frame.
In a TDD scenario, the subframes of a particular frequency are reserved for either uplink transmissions from wireless devices to a serving network node or downlink transmissions from the network node to the wireless devices such that uplink and downlink transmissions do not occur at the same time on the same frequency within the cell. A subframe is basically defined by a preset time period of a certain length, typically 1 millisecond (ms), and each subframe may comprise two time slots of 0.5 ms each. Further, a radio frame comprises a predefined number of consecutive subframes, e.g. ten subframes. In such a network, different network nodes are able to use different uplink-downlink, UL-DL, configurations of subframes, e.g. depending on the current need for uplink and downlink bandwidth resources. An example of a downlink-uplink configuration of subframes is shown in FIG. 1 including downlink subframes “D”, uplink subframes “U” and so-called special subframes “S”. The special subframes S are configured with one part reserved for downlink, another part reserved for uplink, and a guard period with no transmission between the above two parts, thus allowing neither uplink nor downlink in the guard period.
A set of different UL-DL configurations that can be used by network nodes in different radio frames is shown in the table of FIG. 2 comprising seven different UL-DL configurations 0-6 each having ten subframes 0-9 of a repeatable radio frame It can be seen in this example that the first three subframes 0-2 and subframe 5 are reserved for downlink D, special S, uplink U, and downlink D, respectively, in all UL-DL configurations 0-6, while the remaining subframes 3, 4, 6-9 can vary in different UL-DL configurations. The latter subframes 3, 4, 6-9 may thus be referred to as flexible subframes, indicated by “F” in the figure.
In this disclosure, the term “flexible subframe” denotes a subframe in which the direction of transmission, i.e. downlink or uplink, may differ between different cells so that the flexible subframe may be used for downlink in one cell and for uplink in another cell. Further, a flexible subframe may differ from one radio frame to another radio frame in the same cell, so that the flexible subframe may be used in the cell for downlink in one radio frame and for uplink in another radio frame, e.g. depending on the instantaneous traffic demands. Thereby, the link direction of transmissions in flexible subframes over a radio link between a wireless device and a serving network node is not obviously predictable and some amount of signaling is needed to indicate the current direction of a flexible subframe to the served wireless devices. This is commonly referred to as a dynamic TDD scenario.
The network node thus signals the current UL-DL configuration in a suitable manner to any wireless device(s) currently being served by the network node, e.g. in broadcasted system information contained in a System Information Block, SIB, which any wireless device may read, and/or in dedicated signaling, such as Radio Resource Control, RRC, signaling, which is exclusively directed to specific wireless devices which are capable of operating in such a dynamic TDD scenario. The signaling may be done on a per radio frame basis which enables the wireless device to determine in which subframe(s) of the current radio frame it is possible to perform Channel State Information, CSI, measurement of reference signals transmitted by the network node, which is obviously not possible to do in an unsuitable subframe that is configured for uplink transmission according to a current UL-DL configuration. A CSI reference signal is denoted “CSI-RS” which can be transmitted by the network node in a downlink subframe, and the wireless device is then able to perform CSI measurement on the CSI-RS transmitted in that subframe and determine a Channel Quality Information, CQI, value which indicates the quality of the radio link used. The wireless device also reports a specific Channel Quality Information, CQI, value in a CSI reporting subframe to the serving network node, which enables the network node to perform link adaptation based on the CSI report, among other things.
However, it may be a problem that a wireless device is sometimes not able to know which UL-DL configuration is currently used in a particular radio frame, such as when the wireless device has not successfully detected the signaled UL-DL configuration. Consequently, the wireless device may perform CSI measurement in unsuitable subframes which may have changed direction without the wireless device noticing due to missed signaling, which in turn may result in irrelevant and misleading CSI reports and deficient evaluation of channel quality e.g. for the purpose of performing link adaptation in the network node.